Speckle patterns typically occur in diffuse reflections of monochromatic light such as laser light. The speckle effect results from interference of many waves of the same frequency, having different phases and amplitudes, which, together, give a resultant wave whose amplitude, and therefore intensity, varies randomly.
In many laser based displays and sensors, this phenomenon is observed as being the use of coherent light sources with a narrow wave length span. When an image is formed of a rough surface which is illuminated by a coherent light (e.g. a laser beam), a speckle pattern is observed in the image plane.
Speckle is considered to be a problem in laser based display systems (e.g. in laser television and laser projectors) and in laser sensing systems (e.g., LiDAR systems). According to Wikipedia, “speckle is usually quantified by the speckle contrast. Speckle contrast reduction is essentially the creation of many independent speckle patterns, so that they average out on the retina/detector”.
Conventional methods of speckle reduction include:
1. Angle diversity: Illumination from different angles.
2. Polarization diversity: Use of different polarization states.
3. Wavelength diversity: Use of laser sources which differ in wavelength by a small amount.
For example, a SONY picoprojector which uses a Wavelength diversity method to reduce speckles, by providing multiple sources of the same color with variance in wavelength and optical path is described at http://ww.sony.com/electronics/portable-projectors/mp-clla”)
4. Rotating diffusers—which destroy the spatial coherence of the laser light—can also be used to reduce the speckle.
5. Moving/vibrating screens may also be solutions. The Mitsubishi Laser TV appears to use such a screen which requires special care according to their product manual. A more detailed discussion on laser speckle reduction can be found in Kishore V. Chellappan, Erdem Erden, and Hakan Urey, “Laser-based displays: a review,” Appl. Opt. 49, F79-F98 (2010), which is not available to Applicant.
6. Micro-lens array screen (MLA) which controls the diffuser characteristic.
7. Synthetic array heterodyne detection was developed to reduce speckle noise in coherent optical imaging and coherent differential absorption LIDAR.
8. In scientific applications, a spatial filter can be used to reduce speckle.
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